Since lot of player confused about mod types and patches, I give a short list which category what can do:

What can a Visual mod do:
  • Customized things for yourself! (drs, slp, wav, ini)
  • Change an already exists graphics/terrain to something else
  • Change an already exists sound to something else
  • Change an already exists texts to something else
  • Edit player's colors
  • Changing interface looks (uncommon)
  • Adds Random maps or Scenarios

What can a Game mod do:
  • New things + what Visual mods can already for everyone in your room! (.dat + drs, slp, wav, ini)
  • Change/edit units, buildings, objects behaviors, stats, cost, etc
  • New terrain rules (example: ships on land)
  • Edit minimap's colors
  • Change/edit technologies/researches
  • Edit graphics rules (example: unit's attack graphics slower or faster)
  • Sound played at different place (example: wolf howling always at background)
  • Gives new sounds
  • Edit tech tree, civ/team bonuses
  • Gives new civs (a bit complex)

What can a Patch do: